Just as Mitt Romney declared that he's in, it's suddenly looking like he'll have more company in his campaign for the Republican US presidential nomination.

While Romney made his candidacy official in New Hampshire, political heavyweights Sarah Palin and Rudy Giuliani caused a stir of their own with visits to the first-in-the-nation primary state. And rumblings from Texas Governor Rick Perry, New Jersey Governor Chris Christie and Republican Michele Bachmann of Minnesota further undercut Romney's standing as the closest thing the GOP has to a front-runner.

"I'm Mitt Romney and I believe in America. And I'm running for president of the United States," Romney said to cheers on a sunny farm here in Southern New Hampshire.

The former business executive previewed a campaign message focused on the economic woes that top voters' concerns: rising gas prices, stubbornly high unemployment and persistent foreclosures.

"It breaks my heart to see what is happening to this great country," Romney said. "No, Mr. President, you had your chance."

It's a pitch tailored to the conservatives who hold great sway in picking the GOP's presidential nominee in Iowa and South Carolina - and the independents who are the largest political bloc in New Hampshire. And it is as much a statement on his viability as it is an indictment of Obama's leadership.

"Barack Obama has failed America," Romney said as he began his second White House bid. "When Barack Obama took office, the economy was in recession, and he made it worse."

Romney comes to a presidential contest that lacks a true front-runner.

In the last week, the still-forming field became less certain with Giuliani visiting an Italian restaurant here and meeting privately with state activists. In North Conway, Giuliani said he hasn't decided yet if he will run again and that he expects to make up his mind by the end of the summer.

But he certainly sounded like a candidate, telling reporters that the nation is being led in the wrong direction by Obama.

"He's been in office a very long time now and his results on the economy have been abysmal," Giuliani said. "His only answer to it has been, 'Oh, I inherited this.' Well, my goodness, he's been in office long enough now, so that whatever he inherited, he should've straightened out by now."

Palin, her party's 2008 vice presidential nominee, was set to arrive in New Hampshire later today (NZ time) for appearances that highlighted her potential to upend the race should she run. Aides weren't releasing her schedule, but her family's bus tour that rumbled out of Washington last weekend was likely to overshadow the declared candidates.

Perry, too, gave hints he was considering a bid, though his aides sought to tamp down expectations he would join. Tea party darling Bachmann is inching toward a run, perhaps giving the anti-tax, libertarian-leaning grassroots movement a candidate to rally around.

"Who is it that rules this great nation?" Romney said in a nod to tea partyers. "You do."

Embracing familiar conservative rhetoric, Romney said Obama has spent his first three years in office apologizing to the world for the United States' greatness, undercutting Israel and borrowing European-style economic policies. He cast Obama as beholden to Democratic interest groups and indifferent to out-of-work Americans.

"It's time for a president who cares more about America's workers than America's union bosses," Romney said.

He said Obama's policy in Afghanistan was wrong, his spending too high and said his administration sought to seize power through regulation and fiat.

"This president's first answer to every problem is to take power from you. ... And with each of those decisions, we lose more of our freedoms," Romney said.

Romney's strengths are substantial: He's well known and he's an experienced campaigner. He has a personal fortune and an existing network of donors. He has a successful businessman's record.

But his challenges are big, too. They include a record of changing positions on social issues including abortion and gay rights, shifts that have left conservatives questioning his sincerity. He also has struggled to allay some skeptics of his Mormon faith.

Romney oversaw a health care law enacted in Massachusetts that's similar to Obama's national health overhaul, which conservatives despise.

"If I ran through all my mistakes, Ann would love it and you'd be here all night," Romney said, referencing his wife but not explicitly acknowledging the hurdle while calling for a repeal of Democrats' national plan.

His rivals weren't about to let it go. Asked about how big a problem Romney faces regarding the Massachusetts health care law, Giuliani was critical.

"The reality is that Obamacare and Romneycare are almost exactly the same," Giuliani said. "It's not very helpful trying to distinguish them. I would think the best way to handle it is to say, it was a terrible mistake and if I could do it over again, I wouldn't do it."